spidernet: enable support for bcm5461 ethernet phy

A newer board revision changed the type of ethernet phy.
Moreover, this generalizes the way that a phy gets switched
into fiber mode when autodetection is not available.

Signed-off-by: Jens Osterkamp <Jens.Osterkamp@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>